Method of forming an electronic pressure sensitive transducer on a printed circuit board

ABSTRACT

The cost and complexity of an electronic pressure sensitive transducer are decreased by constructing such a transducer directly on a printed circuit board containing support electronics. Conductive traces are formed on the printed circuit board to define a contact area. A flexible substrate having an inner surface is positioned over the contact area. An adhesive spacer, substantially surrounding the contact area, attaches the flexible substrate to the printed circuit board. At least one resistive layer is deposited on the flexible substrate inner surface. In use, the resistive layer contacts at least two conductive traces in response to pressure applied to the flexible substrate to produce an electrical signal indicative of applied pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 60/267,455 filed Feb. 8, 2001 which is incorporated herein byreference. This application is a divisional of U.S. patent applicationSer. No. 10/067,952, filed Feb. 5, 2002, now U.S. Pat. No. 6,909,354.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The Present invention relates to transducer systems for sensingpressure.

2. Background Art

Pressure sensitive transducers generate a signal indicative of theamount of pressure applied to a flexible membrane. Such transducers mayalso generate a signal based on the location of pressure applied to theflexible membrane. Such pressure sensitive transducers provide inputsfor a wide variety of applications such as remote controls, gamecontrollers, mouse pads, tactile sensors, and the like. Pressuresensitive transducers are typically coupled with electronics thatcondition and amplify pressure signals.

Various constructions for pressure sensitive transducers are possible.One type includes one or more force sensing resisters (FSRs). VariousFSRs have been disclosed, such as those described in commonly assignedU.S. Pat. Nos. 4,314,227; 4,314,228; and 4,489,302; each of which ishereby incorporated by reference in its entirety. Typically, an FSR iscomposed of three parts, a rigid base, a spacer, and a resistivemembrane. Conductive traces are typically arranged in separatedinterdigitated sets on the base. These traces may be configured in asingle zone or in multiple zones to allow, for example, pointing devicesas described in commonly assigned U.S. Pat. Nos. 5,659,334 and5,828,363, each of which is hereby incorporated by reference in itsentirety. The flexible resistive membrane is spaced apart from the baselayer by a spacer, which is typically a ring of material around theouter edge of the conductive traces. The spacer is also typically coatedwith adhesive to hold the device together. The flexible top membrane maybe made of a polymer coated on its inner face with semi-conductive orresistive ink, giving the FSR force sensing properties. This ink isdescribed in commonly owned U.S. Pat. Nos. 5,296,837 and 5,302,936, eachof which is hereby incorporated by reference in its entirety.

In most practical applications, the FSR must be connected to sensing andconditioning electronics in order to effectively operate. One way thismay be accomplished is by connecting the FSR to a printed circuit boardcontaining the electronics with a multi-conductor cable. Another way ofconnecting the FSR to support electronics is to adhere the FSR basedirectly to the circuit board containing the electronics. Electricalconnection may be made between traces on the FSR and correspondingtraces on the printed circuit board using z-tape, which only conducts ina direction perpendicular to the tape surface. While either method iseffective, both have unnecessary manufacturing steps and requireunnecessary components, thus increasing the cost of a pressure sensitivetransducer system as well as increasing the likelihood of systemfailure. What is needed is a pressure sensitive transducer and a methodfor making such a transducer that requires fewer components and fewermanufacturing steps without sacrificing transducer performance.

SUMMARY OF THE INVENTION

The present invention decreases the cost and complexity of an electronicpressure sensitive transducer by constructing such a transducer directlyon a printed circuit board containing support electronics.

An electronic pressure sensitive transducer producing an electricalsignal indicative of applied pressure is provided. The transducerincludes a printed circuit board accepting a plurality of electronicelements for processing the transducer electrical signal. Conductivetraces are formed on the printed circuit board to define a contact area.A flexible substrate having an inner surface is positioned over thecontact area. An adhesive spacer substantially surrounds the contactarea. The adhesive spacer attaches the flexible substrate to the printedcircuit board. At least one resistive layer is deposited on the flexiblesubstrate inner surface. The resistive layer contacts at least two ofthe traces in response to pressure applied to the flexible substrate toproduce the electrical signal indicative of applied pressure.

In an embodiment of the present invention, at least one resistive layeris made with resistive ink.

In another embodiment of the present invention, a pedestal is formed onthe printed circuit board substantially around the contact area. Thepedestal receives the adhesive spacer for attaching the flexiblesubstrate. The pedestal increases the space between resistive layers onthe substrate and conductive traces on the printed circuit board. Thepedestal may be formed by coating traces on the printed circuit boardwith a non-conductive material such as soldermask.

In yet another embodiment of the present invention, the conductivetraces include a plurality of sets of traces. Each set of traces isinterconnected within a zone of the contact area. An interconnected setof contact traces extends into each zone. At least one of theinterconnected set of traces may be connected to the electronic elementsfor processing the transducer signal via a through-hole in the printedcircuit board. The through-hole may be within the contact area.

In still another embodiment of the present invention, conductive tracesare arranged in sets of interconnected traces. At least two sets oftraces are interdigitated.

In a further embodiment of the present invention, conductive tracescomprise copper traces covered with an oxidation preventing conductivematerial.

In a still further embodiment of the present invention, conductivetraces comprise screen printed carbon ink.

A method of forming an electronic pressure sensitive transducer on aprinted circuit board is also provided. The printed circuit boardaccepts electronic components for producing signals generated by thepressure sensitive transducer. A plurality of conductive traces areformed on the printed circuit board to form a contact area. At least oneresistive layer is deposited on an inner side of a flexible substrate.The flexible substrate is assembled on the printed circuit board suchthat the flexible substrate resistive layer is facing the printedcircuit board conductive traces. The flexible substrate is held to theprinted circuit board by an adhesive substantially surrounding at leasta portion of the contact area.

A transducer system is also provided. The system includes a printedcircuit board having a plurality of conductive traces. At least two ofthese traces define contact areas. The printed circuit board isconstructed to accept electronic elements for processing electricalsignals produced by a plurality of transducers. Each of these signals isindicative of pressure applied to at least one of the transducers. Thesystem also includes at least one flexible substrate having an innersurface facing each contact area. At least one adhesive spacersubstantially surrounds each contact layer to attach at least oneflexible substrate to the printed circuit board. At least one resistivelayer is deposited on a flexible substrate inner surface. Each resistivelayer contacts at least two of the traces in response to pressureapplied to the flexible substrate to produce an electrical signalindicative of applied pressure. The contact areas, at least one flexiblesubstrate, at least one adhesive spacer and at least one resistive layerform the plurality of transducers, each transducer constructed on theprinted circuit board.

A method of forming an electronic pressure sensitive transducer on aprinted circuit board supporting electronic elements is also provided. Aportion of conductive material on a printed circuit board is selectivelyremoved to define traces in a contact area, traces connecting theelectronic elements to the contact area, and at least a portion of apedestal.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded cross-sectional view illustrating a conceptualizedpressure sensitive transducer according to an embodiment of the presentinvention;

FIG. 2 is a top view of a printed circuit board illustrating a four-zonecontact area according to an embodiment of the present invention;

FIG. 3 is a top view of a printed circuit board illustrating carbontraces according to an embodiment of the present invention;

FIG. 4 is a top view of a pressure sensitive transducer constructedusing the printed circuit board of FIG. 2 or 3 according to anembodiment of the present invention;

FIG. 5 is an exploded view illustrating conceptualized multiplesingle-zone FSRs in a pressure sensitive transducer according to anembodiment of the present invention; and

FIG. 6 is a top view of a printed circuit board implementing multiplesingle-zone FSRs in a pressure sensitive transducer according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 1, an exploded cross-sectional view illustrating aconceptualized pressure sensitive transducer according to an embodimentof the present invention is shown. An electronic pressure sensitivetransducer, shown generally by 20, produces an electrical signalindicative of applied pressure. Pressure transducer 20 includes printedcircuit board 22 accepting a plurality of electronic elements, not shownfor clarity, for processing the transducer electrical signal. Conductivetraces 24 are formed on printed circuit board 22 to define contact area26. Flexible substrate 28 has inner surface 30 which faces contact area26 when pressure transducer 20 is assembled. At least one resistivelayer 32 is deposited on inner surface 30. Adhesive spacer 34substantially surrounds contact area 26. Adhesive spacer 34 attachesflexible substrate 28 to printed circuit board 22. When assembled,resistive layer 32 contacts at least two traces 24 in response topressure applied to flexible substrate 28 to produce electrical signalsindicative of applied pressure.

Flexible substrate 28 with resistive layer 32 together with traces 24 onprinted circuit board 22 implement a force sensing resistor (FSR), showngenerally by 36. Integrating FSR 36 directly onto printed circuit board22 creates a pressure sensor transducer which may be referred to as“sensor on board” (SOB). Such a construction eliminates some of thematerials and manufacturing steps previously required to manufacture apressure sensitive transducer.

Virtually any printed circuit board 22 can be adapted to receive FSR 36according to the present invention, providing that sufficient surfacespace is available for FSR 36. This includes both rigid and flexiblecircuit boards. Conductive traces 24 on printed circuit board 22 may beformed by any suitable means known in the art. For example, traces 24may be formed by depositing conductive material on printed circuit board22 then selectively removing a portion of the conductive material todefine traces 24. Dimensions for conductive traces 24 depend on thedimensions of FSR 36, material and construction for flexible substrate28, material and construction for resistive layers 32, and the like.Typical line thicknesses for conductive traces 24 range between 0.010inches (0.25 mm) and 0.060 inches (1.5 mm). Typical line spacing betweenconductive traces 24 ranges between 0.010 inches (0.25 mm) and 0.060inches (1.5 mm). Positional tolerance of FSR 36 on printed circuit board22 varies as well, with ±0.015 inches (±0.4 mm) typical.

Two sets of interconnected traces 24 may be used to form a single zonewithin contact area 26. Multiple zones within contact area 26 permitlocation of pressure on flexible substrate 28 to be determined. Multiplezones may be obtained by using a plurality of sets of interconnectedtraces 24 and one interconnected set of common traces 24 extending intoeach zone. Alternatively, each zone may be defined by two or moreseparate sets of traces 24. Preferably, the sets of traces 24 in eachzone are interdigitated.

Flexible substrate 28 may be constructed from any suitably flexiblematerial such as, for example, Mylar. The thickness of substrate 28varies depending on the application and dimensions of FSR 36. Preparingsubstrate 28 for resistive layers 32 entails cutting material forsubstrate 28 into sheets or other shapes suitable for printing or othermethods of depositing resistive layers 32.

One or more resistive layers 32 are deposited on substrate 28 byconventional means such as, for example, by screen printing resistiveink onto substrate 28. Resistive layers 32 may be printed over theentire substrate 28 or only over that portion which will cover operativetraces 24 on printed circuit board 22. Resistive layers 32 may also bedeposited such that several distinct regions are formed over thatportion of substrate 28 which will cover operative traces 24 on printedcircuit board 22.

Adhesive 34 is used to attach flexible substrate 28 to printed circuitboard 22. Adhesive 34 also provides spacing between resistive layers 32on substrate 28 and traces 24 on printed circuit board 22. Adhesive 34may be applied to printed circuit board 22, to flexible substrate 28, orto both as suits manufacturability of FSR 36. Adhesive 34 may be appliedto either surface in a conventional manner such as, for example, bydepositing a bead of adhesive 34 around some or all of the perimeter ofcontact area 26. In a preferred embodiment, adhesive layer 34 comprisesan adhesive ink screen printed onto substrate 28. Adhesive inks that maybe used include product numbers SP-7533 from 3M or ML25184 from AchesonIndustries, Inc. of Port Huron, Mich.

If adhesive 34 is applied to substrate 28 in advance of final assembly,a protective release liner may be cut or positioned over adhesive 34 toprevent inadvertent adhesion to other surfaces or airborne materials.

Substrate 28 may be formed into strips and kiss cut, or partially cut,into the desired final shape. Individual substrates 28 may then beeasily separated by hand or machine immediately prior to assembly ontoprinted circuit board 22. Preferably, this final assembly step occursafter electronic components have been mounted on printed circuit board22 to prevent heat damage to substrate 28 from soldering operations.

If additional space is required between printed circuit board 22 andsubstrate 28, a pedestal, shown generally by 38, may be formed onprinted circuit board 22 in regions where substrate 28 is adhered toprinted circuit board 22. Pedestal 38 increases the distance thatsubstrate 28 must be depressed prior to contact between resistive layer22 and traces 24 on printed circuit board 22. Pedestal 38 may be formedin a variety of manners. First, printed circuit board 22 may bemanufactured with additional thickness to form pedestal 38. Second, oneor more layers of conductive traces 40 may be built on printed circuitboard 22 to form pedestal 38. Pedestal 38 is then covered with anon-conducting material 42, such as soldermask, to prevent inadvertentshort circuits between traces 40 and resistive layer 32. Third, apolymer thick film may be deposited on printed circuit board 22 to formpedestal 38. Fourth, a sheet of material may be adhered to printedcircuit board 22 to form pedestal 38. As will be recognized by one ofordinary skill in the art, many constructions for pedestal 38 arepossible.

Referring now to FIG. 2, a top view of a printed circuit boardillustrating a four-zone contact area according to an embodiment of thepresent invention is shown. Printed circuit board 22 includes traces 24within contact area 26 forming four zones, each shown generally by 50.Each zone 50 includes one set of traces 24 interconnected by line 52which extends out of contact area 26 to through-hole 54. Through-hole 54permits traces 24 to be connected to electronics mounted on the bottomside of circuit board 22 by soldering processes known in the art. Traces52 may also connect traces 24 to electronic elements on the same side ofprinted circuit board 22 as traces 24. Each zone 50 may also share acommon set of interconnected traces joined to electronics on the backside of circuit board 22 via through-hole 56 in the center of contactarea 26.

Traces 24 may be formed by any means which presents a conductive surfaceto resistive layer 32. Since air typically fills the gap between printedcircuit board 22 and flexible substrate 28, traces 30 should resistcorrosion. Traces 24 may be constructed, for example, by plating orcoating copper traces with an oxidation preventing conductive materialsuch as gold, silver, solder, carbon ink, and the like. Alternatively,traces 24 may be constructed by screen printing carbon, silver, or otherconductive inks onto printed circuit board 22.

Traces 24 and at least a portion of pedestal 38 may be fabricated at thesame time and of the same materials. This may result in traces 24 andthe base of pedestal 38 extending the same height above board 22. Theheight of pedestal 38 may be increased by the addition of solder mask orsimilar material and adhesive spacer 34. This typically will result in adistance between traces 24 and flexible substrate 28 sufficient toprevent inadvertent contact between resistive layer 32 and conductivetraces 24. Simultaneous cofabrication of pedestal 38 and traces 24together with the remainder of circuit board 22 results in very lowincremental cost to circuit board 22.

Referring now to FIG. 3, a top view of a printed circuit board withcarbon ink traces according to an embodiment of the present invention isshown. Printed circuit board 22 includes screen printed carbon inktraces 24 within contact area 26 forming four zones 50. Due to the highresistivity of screen printed carbon ink, traces 24 are printed overcopper pads 60 located just outside of contact area 26. Copper traces 62connect pads 60 to through holes 54 or to electrical components on thesame side of printed circuit board 22 as traces 24.

Referring now to FIG. 4, a top view of a pressure sensitive transducerconstructed using the printed circuit board of FIGS. 2 or 3 according toan embodiment of the present invention is shown. Flexible substrate 28is shown adhered to pedestal 38 on printed circuit board 22. Thisresults in FSR 36 directly constructed on printed circuit board 22.

Referring now to FIG. 5, an exploded view illustrating multiplesingle-zone FSRs in a pressure sensitive transducer according to anembodiment of the present invention is shown. Six FSRs are shown. Eachcontact area 26 includes two sets of interdigitated contacts 24. Spacer38, in this example a thin plastic sheet coated with adhesive on bothsides, is adhered to printed circuit board 22. Spacer 38 includes anopening for each contact area 26. Flexible substrate 28 is then adheredto spacer 38. The entire inner surface 30 of flexible substrate 28 maybe coated with one or more resistive layers 32. Alternatively, separatedisconnected resistive layers 32 corresponding with each contact area 26may be formed on flexible substrate 28.

Referring now to FIG. 6, a top view of a printed circuit boardimplementing multiple single-zone FSRs in a pressure sensitivetransducer according to an embodiment of the present invention is shown.Printed circuit board 22 includes six contact areas 26 with traces 24formed by screen printing carbon ink onto printed circuit board 22.Copper pedestal 38 surrounds contact areas 26. Pedestal 38 may have awide variety of shapes, but typically mirrors the outline of flexiblesubstrate 28, not shown for clarity. Soldermask 42 covers much ofcircuit board 22 while leaving contact areas 26 exposed to contactresistive layer 32 on flexible substrate 28. Printed circuit board 22also contains electronic components, shown generally by 70, soldered toprinted circuit board 22 for receiving electrical signals from traces 24indicative of pressure applied to the FSRs.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A method of forming an electronic pressure sensitive transducer on a printed circuit board, the printed circuit board accepting a plurality of electronic components for processing signals generated by the pressure sensitive transducer, the method comprising: forming a first plurality of conductive traces on the printed circuit board to form a contact area; forming a pedestal on the printed circuit board substantially around the contact area, with a second plurality of conductive traces the second traces formed on the printed circuit board in the shape of the pedestal and covered with a non-conducting material; depositing at least one resistive layer on an inner side of a flexible substrate; and assembling the flexible substrate on the printed circuit board such that the flexible substrate resistive layer is facing the printed circuit board conductive traces, the flexible substrate held to the printed circuit board by an adhesive substantially surrounding at least a portion of the contact area and contacting the pedestal.
 2. The method of forming an electronic pressure sensitive transducer on a printed circuit board as in claim 1 wherein at least one resistive layer comprises resistive ink.
 3. The method of forming an electronic pressure sensitive transducer on a printed circuit board as in claim 1 wherein assembling comprises screen printing the adhesive on at least one of the flexible substrate and the printed circuit board.
 4. The method of forming an electronic pressure sensitive transducer on a printed circuit board as in claim 1 wherein the traces forming the pedestal are formed in the same process as the traces forming the contact area.
 5. The method of forming an electronic pressure sensitive transducer on a printed circuit board as in claim 1 wherein the first plurality of conductive traces are formed as a plurality of sets of zonal traces, each set of zonal traces interconnected within a zone of the contact area, and as an interconnected set of common traces extending into each zone.
 6. The method of forming an electronic pressure sensitive transducer on a printed circuit board as in claim 1 wherein forming a first plurality of conductive traces comprises depositing an oxidation preventing conductive material over copper traces.
 7. The method of forming an electronic pressure sensitive transducer on a printed circuit board as in claim 1 wherein forming a first plurality of conductive traces comprises screen printing a carbon ink. 